Focus in talk was primarily on BaFe_2(As_1-xP_x)_2 (P-doped 122). This system is interesting and unusual because a) it shows transport properties suggestive of quantum critical behavior near optimal doping at P=0.33; b) the SC dome is created by “doping” with P, which is however isovalent to As; c) the superconducting state properties are strongly suggestive of line nodes. Here are resistivity measurements from Matsuda group quite reminiscent of cuprates showing relatively large range linear fit to resistivity at x=0.33, and deviations away from this doping.
P-doping affords the possibility for NMR to do very clean experiment, since ^31P nucleus is spin-1/2 & eliminates quadrupole contribution to signal. At first glance the symmetric position of the As or P above Fe plane might make you think it would not be sensitive to AF correlations, but Ishida showed that this is incorrect when one takes into account full structure of hyperfine interaction tensor. Below, for example, is shown the expected response in the presence of certain assumed type of order on the Fe atoms (red).
Ishida said by changing angle of field, and noting that the experiment is sensitive to the fluctuations transverse to the field direction, they can verifiy that the order and low-frequency fluctuations are indeed of the stripe type shown in 1st panel above.
Here are Ishida’s spin-lattice relaxation rate (T_1) data for various P dopings (Nakai et al., to appear in PRL). Ishida noted the large upturn in the (T_1T)^(-1) signal in the underdoped (magnetic) materials just above optimal doping. These fluctuations appear to grow until, near optimal doping, they reach a maximum (at this point T_N
And T_c are nearly equal— Ishida said whether they cross is not directly clear from NMR, and there is no neutron work because crystals are too small).
Data were fit to Curie-Weiss term b/(T+theta) plus constant offset. Variation of Curie-Weiss theta with doping shows theta goes through zero at/near optimal doping, where effective mass of one of the orbits from dHvA also appears to diverge. Ishida: this is consistent with claimed QCP at or near optimal doping. Alloul: there is no evidence for quantum cricitcal *behavior* from NMR alone; fact that theta-->0 at x=0.33 merely indicates T_N going to zero. Chubukov: is there low-T data showing QCP or only high-T extrapolated? Ishida: only the latter.
Ishida: qualitative behavior of Co-doped system is very similar, again with theta crossing zero around optimal doping. Some differences with K-doped material could be understood by changes in band structure, DOS at Fermi level which decreases as one e-dopes. Ishida reminded us that this emphasizes one great advantage of P-doping on the As site: unlike cuprates, or Co,K-doped 122 systems, changing “dopant” P does not change the DOS and one can examine changes in spin dynamics independently. Evidence: relative x independence of Knight shift.
Fernandes asked if there was evidence for re-entrant behavior of the superconductivity near the magnetic transition as in Co-doped systems. Ishida said he was not aware of any.
While the focus of this talk was not on SC state, Ishida pointed out that near optimal doping T-dependence was close to T^3 with low-T linear term (consistent with line nodes and some dirt). Benfatto pointed out that the overdoped data do not show the expected decrease of (T_1T)^(-1) below Tc at all, and Ishida said this was not understood.
Comparison with La-1111 (As NMR, powdered samples). Ishida pointed out that 1/T_1T changes by 2 orders of magnitude with F doping over a range x=0.05-0.15 where Tc changes by less than a factor of 2. Data show that while a similar, if weaker, increase in spin fluctuations occurs just above T_N in the magnetic phase, when one reaches the superconducting F doping concentrations (recall T_N has a first order drop to zero with doping in this system) these fluctuations are gone and (T_1T)^-1 is flat with T. His group is trying hard to understand these differences with 122 systems. Ishida claimed other 1111 systems, including those with higher Tc, show similar behavior when one subtracts rare earth magnetism (Ce,Sm, …).
Hirschfeld asked “devil’s advocate question” : if higher Tc superconducting family 1111 shows weaker spin fluctuations above the transition at optimal doping, is this not evidence that spin fluctuations are NOT responsible for superconductivity, or at least that another mechanism may be in play? Ishida agreed this was an important open question that they are investigating.
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